Metals are commonly cast as ingot by pouring molten metal into one end opening of an open ended mold while the resulting body of partially solidified metal or "ingot" is advanced from the opposing end of the mold on a stool or support which is reciprocated in relation to the mold. To cast successfully, however, the operator must closely control the temperature of the metal, and this is accomplished by cooling the mold itself, and directing liquid coolant against the surface of the metal ingot as it emerges from the mold. The rate at which heat is extracted from the metal by the latter operation is a function of the temperature of the coolant itself, and the velocity of the coolant flow. For any given piece of molding equipment, moreover, the velocity is largely a function of the rate at which the coolant is discharged onto the ingot.
Initially, both the metal and the equipment are relatively cold, and therefore, the support is reciprocated from the mold at a relatively low rate of withdrawal or "casting speed." The coolant is likewise discharged at a relatively low rate, and every other attempt is made to maintain a low rate of heat extraction from the ingot while the butt end of the same is being formed on the support. However, after the butt end has emerged from the mold, the casting speed is increased, and for the remainder of the casting operation, the coolant is discharged onto the ingot at a sharply increased rate. This latter stage is commonly called the "steady state" casting stage. The initial low speed casting stage is commonly called the "butt forming" stage.
Unfortunately, plant operators have not been able to control the parameters of coolant temperature and velocity to the extent they would like. The coolant is commonly the water supplied to the operator's plant from local sources, and not only does the supply fluctuate as to available volume, but it fluctuates dramatically in temperature from one season to another, such as from summer to winter and vice versa. Furthermore, there is a minimum flow rate which the operator must maintain if he is to avoid a point where so called "film boiling" occurs. This is the point at which the surface of the ingot is no longer continuously wetted by the coolant, but instead is enveloped in a film of steam which limits heat loss from the metal to the factors of conduction and radiation alone. Often, when the local water is too hot and/or in short supply, operators have had to import additional water to lower the temperature of the coolant and maintain a cooling rate above that at which film boiling would occur.
The patentee in U.S. Pat. No. 4,166,495 sought to control the rate of heat removal during the initial low speed butt forming stage of his operation by dissolving a gas in the coolant. The addition of the gas was said to retard the rate at which heat was extracted from the metal during this initial stage. Later, when the steady state casting stage was begun, the gas was no longer dissolved in the coolant, and the operation was conducted thereafter with the coolant alone.